linux-kernel - Re: [PATCH v4 0/8] x86/bugs: KVM: L1TF and MMIO Stale Data cleanups

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Message-ID: <DDWGVW5SJQ4S.3KBFOQPJQWLLK@google.com>
Date: Fri, 31 Oct 2025 11:22:45 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>
Cc: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v4 0/8] x86/bugs: KVM: L1TF and MMIO Stale Data cleanups

On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> This is a combination of Brendan's work to unify the L1TF L1D flushing
> mitigation, and Pawan's work to bring some sanity to the mitigations that
> clear CPU buffers, with a bunch of glue code and some polishing from me.
>
> The "v4" is relative to the L1TF series.  I smushed the two series together
> as Pawan's idea to clear CPU buffers for MMIO in vmenter.S obviated the need
> for a separate cleanup/fix to have vmx_l1d_flush() return true/false, and
> handling the series separately would have been a lot of work+churn for no
> real benefit.
>
> TL;DR:
>
>  - Unify L1TF flushing under per-CPU variable
>  - Bury L1TF L1D flushing under CONFIG_CPU_MITIGATIONS=y
>  - Move MMIO Stale Data into asm, and do VERW at most once per VM-Enter
>
> To allow VMX to use ALTERNATIVE_2 to select slightly different flows for doing
> VERW, tweak the low lever macros in nospec-branch.h to define the instruction
> sequence, and then wrap it with __stringify() as needed.
>
> The non-VMX code is lightly tested (but there's far less chance for breakage
> there).  For the VMX code, I verified it does what I want (which may or may
> not be correct :-D) by hacking the code to force/clear various mitigations, and
> using ud2 to confirm the right path got selected.

FWIW [0] offers a way to check end-to-end that an L1TF exploit is broken
by the mitigation. It's a bit of a long-winded way to achieve that and I
guess L1TF is anyway the easy case here, but I couldn't resist promoting
it.

(I just received a Skylake machine from ebay, once that's set up I'll be
able to double check on there that things still work).